Monday, March 23, 2009

I woke up before noon to consider whether to prepare for a chase. Not unlike many set-ups while I'm on nights... I'm still sitting at home, considering going back to bed to get some more sleep!

Color me surprised, but convective initiation has already occurred along the dryline, in an arc from Brady NE east-southeast to Kearney NE and then south-southeast to Osborne/Lincoln, KS. In hindsight, this is likely a result of deep mixing in the moist sector with sfc temps rising well through the 70s (5-10F warmer than forecast). Deep convection is also trying to form farther south, from Medicine Lodge KS down toward Clinton OK... though its been unsuccessful thus far... probably will have a better shot as mid-level cooling/lift continues to overspread the dryline.

For the northern stuff, surface obs show T/Td spreads of 22-25F... so for now the tornado threat would seem to be low.

Sfc winds are sustained at 39 kt at Hutchinson KS! Local WFOs have issued high wind warnings for a good chunk of central KS.

This is just as tough a forecast as expected, so my confidence is pretty low on the details. If the mid to upper 50s dewpoints can back northwestward up to the dryline and discrete convection develops and persists, still could certainly be some tors southcentral KS/northcentral OK, mainly near/after 00Z as the boundary layer decouples.

I don't have time to get too in-depth with this... actually haven't even looked at as much data as I'd have liked to. I may try to chase after a 5-hour sleep following work this morning, maybe targeting the dryline in southcentral KS. Here are some random thoughts for tomorrow.

-Even if the pseudo-cold core play (occluded/cold front in NE) was within reach, I'm still not sure I see it as being better than the dryline play. I don't see a baroclinic boundary, and while surface winds may be isallobarically backed up there, low-level shear doesn't look any better than points south given a more backed storm motion and the likelihood of a weaker LLJ. Still, wouldn't be surprised to see a few tornadoes up there (especially if they heat up well into the 60s°F)... though my hunch is that convection will assume the form of line segments. One thing that is better about the N target is a very high chance of daytime initiation, though.

-The first thing that concerns me about the dryline target is the extremely strong surface pressure gradient across southcentral and eastern KS... surface winds are no doubt being underforecast by the models, and that could easily be somewhat of an SRH-killer tomorrow afternoon. As I alluded to in previous posts, 30 kt sustained winds are generally "not your friend" on a chase day. The NAM-WRF indicates the pressure gradient and surface winds should begin to relax by 00Z and more certainly by 03Z, which should naturally allow for an increase in low-level shear. My hope is that sr-hodographs as forecast (250-350 m2/s2 of 0-1 km SRH, with rightward deviance not having a whole lot of net effect on the SRH magnitude in this event) are still large enough such that stronger-than-forecast surface winds still allow for fairly large SRH (> 200 m2/s2).

-Another thing that concerns me about the dryline play, particularly given that I work tonight, is the timing of convective initiation. Interestingly, the 00Z NAM-WRF weakens the forward-side-of-trough 5H wind max between 18Z and 00Z as it rotates atop the KS dryline... fragmenting the vorticity into a piece north (closer to the mid-level low center) and a trailing piece back in the dry air. The attendant vertical motion field is similarly fragmented, not organizing over and then intercepting the dryline until after 00Z... and I have to wonder if it is related to the weakening of the mid-level jet. (I've had a good number of chases in the past, some involving a cold core and some not, where I've been left high and dry under the mid-level jet max... while initiation and tornadoes occur farther N, just east and southeast of the primary vort center). High-res simulated radar reflectivity from both the 00Z NMM and ARW don't initiate dryline convection until between 00Z and 03Z, either, and the 00Z and 06Z operational NAM-WRF and 03Z NAM-KF all indicate initiation starting right around 00Z (7 P.M. ... not really worth trying to chase given my working tonight). It's anyone's guess whether this guidance will come to fruition as described, but regardless, it is supported by the background mass fields and overall it lessens my confidence in getting storms to pop earlier rather than later. One would think that with a strongly convergent dryline and lifting/cooling of the residual cap from the west... initiation would be possible by 4-6 P.M. We shall see. We certainly can't have our moisture mixing out if we want to achieve dryline initiation. Which leads me to...

-My last concern, obviously, is the magnitude of moisture. The 00Z NAM-WRF forecast is a bit better than prior runs, indicating surface dewpoints solidly in the upper 50s°F over southcentral KS by peak heating. 100mb ML dewpoints on the 00Z RAOBs were already around 50°F, so we don't have far to go... with the 55°F surface isodrosotherm about near the Red River as I type. I worry about the effects of the strong vertical mixing, though, and whether we will end up with poorer boundary layer moisture than is forecast. It certainly isn't a good sign that boundary layer moisture didn't improve any over deep south TX during the last 24 hours, per the 00Z Brownsville and Corpus Christi RAOBs.

One final thing which will be interesting is the evolution of convection as the plunging cold front overtakes the dryline, which wasn't addressed much in the otherwise excellent 06Z SPC Day 1 Outlook. The large scale pattern bares some resemblances to May 1, 2008... with an extremely intense band of high-level difluence/divergence forecast to overtake the sharpening cold front between 03Z and 12Z. Could this lead to another very damaging QLCS with embedded circulations/F3 tornadoes? Hopefully not, but an organized squall line with some damaging gusts seems like a good bet.

It's also good to see that an extreme snow event is looking less likely for central ND, as the upper low is now forecast to be weaker (with filling low-level cyclones) as it lifts northeast and gradually opens into a shear axis. My family back home is dealing with flooding due to tons of melting snow already, and they really don't need another 18" on top of it. Hopefully they skate by with 6-8" in Bismarck.

Saturday, March 21, 2009

Just a dismal forecast update for Monday's severe weather potential. It's really too bad we don't have another day of return flow ahead of this system, or rather, that trajectories don't improve sooner than forecast. It looks like the boundary layer air mass over the southern two-thirds of the Gulf of Mexico has modified further, with moisture deepening considerably. The Brownsville TX 00Z RAOB looked far better than I would have expected, with a 1.5 km deep moist layer and a 100mb ML dewpoint of 63°F! In fact, dense MVFR stratus has already materialized over portions of the Rio Grande valley and up as far north as the TX Hill Country. But none of this seems to matter, as our trajectories just aren't forecast to improve in time to get low-level moisture of anywhere near that quality into the central Plains prior to darkfall Monday. I'll do one last forecast tomorrow night when the new data is in... with the system being onshore the models should have a very strong lock on it by then. The 00Z GFS tonight still seems to be having issues with keeping the warm sector socked in with clouds and cooler temps. Anyway, my hunch is that this could be the type of system where dewpoints on the north end of the warm sector aren't terribly weaker than well to the south... e.g. broad mid 50s°F at the surface. If so, though again the impressively cold 7H-5H air will probably be lagging the warm sector a bit, there could still be some enhancement to the low-level destabilization process up north. Any cold core potential looks like a "warm sector" variety up there, as a synoptic warm front will not be present (rather just an inverted trough extending N of the surface low). That target is out of my reach regardless, but it will still be interesting to watch how that area may evolve... especially w.r.t. low-level destabilization and ultimate convective mode. The NAM is actually showing strong crossover of the 0-6 km bulk shear vectors with the bent back/occluded front up there, though they back with time. The models are really still all over the place on many mesoscale facets of this system though, and hopefully tomorrow night things will look clearer.

Using the term "forecast" loosely, as I'm at the end of a mid shift and just coherent enough to throw together a discussion...

The first potential severe weather maker in the Plains in the past several weeks (aside from the bird fart of an event on March 9) is pretty clearly going to arrive on Monday. While many local and even national media outlets have, oddly, been advertising the potential for a considerable tornado outbreak, I remain unimpressed at worst and "mildly interested" at best w.r.t. tornado potential.

A large upper level trough, with 500 mb temps of -36 to -40°C, is currently over portions of the northern Pacific and Gulf of Alaska... with two embedded closed lows apparent off the coast of British Columbia and a third fast-moving perturbation ejecting into the forward side of the large scale trough. Digging mid- and high-level jets on the trough's back side will continue to carve out the trough as it consolidates southeastward into much of CA and the CA coastal waters Sunday night... and then turns east toward the central plains on Monday. The ECMWF and GFS are coming into very close agreement on mass fields with the 00Z run of each model. Expect a mid-level low to close off over eastern CO by 12Z Monday within the neutral tilt rough, and then lift northeast into western NE by 00Z... with an occluding, sub 990 mb surface low preceding it. The synoptic cold front will begin plunging through western NE and into northwestern KS during the day, probably forming a triple point with the occluding dryline in southcentral NE and far northcentral KS by afternoon.

A plethora of questions exist as they relate to the ultimate magnitude and type of severe weather. They include:

- Moisture/instability: although 65-72°F surface dewpoints are still hanging on essentially across the southwestern half of the Gulf of Mexico, the moisture is likely very shallow. Moreover, the sprawling surface ridge over much of the eastern seaboard and deep South will not budge a lot the next 48 hours, which means easterly trajectories persisting across the Gulf of Mexico... and with warm boundary layer air over the same areas, considerable moisture flux from the gulf would seem unlikely. Trajectories won't become more favorable for northwestward advection of higher moisture until sometime Sunday night. The GFS/ECMWF/NAM all agree on 100 mb ML dewpoints of 54-56°F spreading into central KS and central OK in advance of the dryline by Monday afternoon... which considering the above, looks reasonable. This is pretty poor moisture quality for a "warm sector" event in late March. The GFS generates/advects some very cold air atop the BL by 00Z (resulting in wickedly large low-level CAPE), which reeks of convective feedback. Also, the GFS keeps the warm sector socked in with low clouds and cool surface temps. Preferring the ECMWF/NAM thermodynamics, would expect at least filtered March sun ahead of the dryline to allow temperatures to climb to near 70°F. Although the impressively cold upper tropospheric air will lag well to the west, 0-2°C at 700mb overtaking the dryline by mid-afternoon should aid in eliminating the cap, with MLCAPE approaching 1000 J/kg and modest amounts of low-level CAPE.

- Storm mode / tornado threat: A strongly convergent dryline should evolve by mid-afternoon given the strength of the low-level cyclone and intense downward mixing of westerly momentum in the high plains. This coupled with strong DCVA on the inside of the mid-level jet max would seem to point toward fairly vigorous/widespread convective initiation in a narrow band from north to south. 0-6 km bulk shear vectors on the ECMWF have consistently shown a healthy crossover with the dryline (45-60 degrees), while on the 00Z GFS the angle is a bit smaller (though still respectable). Generally expect a mix of line segments and discrete cells given the favorable shear vector orientation coupled with very strong synoptic scale/mesoscale lift and modest instability. Given the ambient surface winds will probably be quite strong (25 kts or more), the 50-60 kt LLJ maintained by the ECMWF/NAM would be preferred in order to maintain strong low-level shear for a substantial tornado threat. The 00Z GFS hodographs looked a bit poorer than in previous runs with the primary LLJ core shifting east away from the dryline, though the 06Z run looks better again. Most of my experiences with very strong ambient surface winds have been bad ones as they are notorious for putting a big kabosh on low-level shear... but given a 50-60 kt LLJ is maintained during the afternoon and storms are successful in becoming rightward-deviant, that won't be an issue on Monday. We'll see.

As usual, mesoscale details should be clearer by the morning of the event. My biggest hope is that KC can get some more rainfall out of this one...